Using radiolabeled fatty acids, a mathematical model was developed to calculate transfer constants (k*FFA) and rates of incorporation (J-FFA) of plasma free fatty acids into individual brain regions. 3H-Palmitic acid (PA) is incorporated into the sn-1 position of phosphatidylcholine; (14)C-arachidonic acid (AA) is incorporated into the sn-2 position of phosphatidylinositol and phosphatidylcholine; (14)C-docosahexaenoic acid (DHA) is incorporated into the sn-2 position of phosphatidylethanolamine. The incorporation of 3H-AA and (14) C-DHA, but not 3H-PA, into brain lipids increased in rats give arecoline a cholinergic agonist. Regional increases were greater in brain areas with M1, receptors which are linked to phospholipid turnover. Incorporation of 3H-PA into intracerebrally implanted Walker 256 carcinosarcona was 3-6 fold higher than control brains in rats. Incorporation of (18)F-dl-erythro-9, 10-difluoropalmitate into brain phospholipids was similar to that of (14)C-PA. Therefore, this tracer may be a useful probe for studying brain livid metabolism in humans with positron emission tomography.